It was realized early in the development of flow cytometry that the angular dependence of the light scattering intensity from a particle contains a wealth of information related to a particle's intrinsic and extrinsic properties. For example, Mullaney, et al. experimentally demonstrated the use of forward light scatter (0.5-2 degrees) for cell size estimation. In the same time period, it was also realized that cell orientation relative to the incident light beam can introduce artifacts that affect population resolution and instrument sensitivity for particles that do not possess axial symmetry parallel to the flow direction. Loken et al. showed that nonspherical particles in the form of fixed chicken erythrocytes yield a bimodal scattering distribution that is related to rim-on or face-on illumination of the disk-cell structure. Particle orientation dependent scattering effects due to asymmetries that are apparent in the complex geometrical shape of sperm cells have also been demonstrated. Several researchers have suggested passive solutions to orienting particles by shaping the sample nozzle to introduce asymmetries into the velocity flow field of the hydrodynamic sheath system. It has been shown that the most critical aspect in efficient X and Y sperm discrimination in flow cytometric sorting is the orientation of the sperm in the optical scattering plane. Recently, novel nozzle geometries have demonstrated proper orientation of up 60% of the sperm heads in the optical scattering plane at analysis rates near 2000 particles/s dramatically affecting the sorting efficiency. Other researchers have addressed particle asymmetries by collecting data over a large array of scattering angles using scanning flow cytometry (SFC) at the cost of lower particle analysis rates (approximately 400 particles/s). Systems have demonstrated continuous angular scattering data spanning 70 degrees but the data is taken relative to an arbitrary particle orientation that results in large variations of the collected angular spectra for asymmetric particles.
One of the earliest large-scale demonstrations of separating biological cells using acoustic forces was done by Doblhoff, et al. In that system, acoustic radiation pressure was used for selective retention of viable hybridoma cells for the purpose of removing nonviable cells and smaller cellular debris from a 20 liter bioreactor. That system was based on a multi-planar transducer design and demonstrated viable cell retention rates as high as 99.5% with varying results for cellular debris rejection. That early system required high power input (in excess of 15 W) thus necessitating a cooling unit for the drive transducers. More recently, Feke and coworkers developed a novel particle separation strategy that relies on both acoustic radiation pressure and secondary acoustic forces. A high-porosity polyester mesh (pore size two orders of magnitude greater than particle size) in an acoustic standing wave served as a collection matrix whereby particles at nodal locations were trapped within the mesh and secondary acoustic forces formed particle agglomerates and created an attractive force at the mesh surface. In a similar demonstration of retention of hybridoma cells, retention efficiencies of about 95% were achieved with negligible effects on cell viability. This system achieved high cell densities of approximately 1.5×10 8 cells/mL with only hundreds of milliWatts of input power.